The present invention relates to the field of microwave heating apparatus, and more particularly to microwave heating apparatus designed for heating materials in a container. More particularly, the invention relates to apparatus and a method for heating, melting, and solidifying waste materials, especially radioactive wastes. Most particularly, the present invention embodies a system for solidifying transuranic aqueous precipitation sludge by sintering or melting the waste to form a solid monolithic product using microwave technology.
In the art of microwave heating, materials to be heated are generally placed in containers that are, in turn, placed in a resonant cavity into which microwave energy is directed. The containers, themselves, are made from materials that are substantially transparent to microwave energy. A pertinent prior art patent is U.S. Pat. No. 4,330,698 of Sawada et al.
In the patent of Sawada et al., a process for treating a waste material is disclosed. A metal crucible is placed inside a detachable lower half of a resonant cavity. The detachable section is then lifted up to couple with the top section. A rotating shaft and table penetrate the lower section of the cavity to continuously turn the crucible. The material that is heated is moved continually through the microwave field. Consequently, large variations in reflected microwave power occur. A complicated continuously moving tuner is employed in order to minimize the reflected power due to the variations in the waste material surface.
In the Sawada et al. process, offgas and dust are removed from the system in the upper section of the cavity directly opposite of the microwave energy waveguide input. As a result, the residence time of the offgas and dust in the resonant cavity is relatively long thereby increasing the chance of ionization of the gas occurring.
The complexity of the resonant cavity of Sawada et al. makes it desirable to design a resonant cavity which allows easier access to the system than is obtained by Sawada. The complexity of the tuner required in the Sawada et al. system and process makes it desirable to provide a microwave heating system and process that does not require such a complex tuning system. The relatively long residence time of offgases in the Sawada et al. system makes it desirable to provide a microwave heating system that sweeps out offgases more rapidly.
Turning now to a specific waste disposal problem, one specific problem of utmost importance is the disposal of radioactive wastes. More specifically, process water in the nuclear industry may contain radioactive transuranic isotopes. A process for removing these wastes from the water and concentrating them involves a step employing aqueous hydroxide precipitation. As a result of this step, the transuranic isotopes are present as a solid hydroxide or oxide form in a water slurry. It would be desirable to trap and concentrate the waste hydroxides and oxides in the slurry to further reduce the volume they occupy. Furthermore, it would be desirable to transform the waste products from an aqueous slurry into a substantially dry product.
Microwave technology has been used in the food and chemical industries since early 1970, with the majority of the work concentrated in the area of drying and the vulcanization of rubber. High-temperature technology has been developed by the Japanese for converting plutonium nitrate, recovered from spent fuel reprocessing, to plutonium oxide for nuclear fuel production, as disclosed in "Continuous Denitration Test Equipment Using Microwave Heating", by Hirofumi Wshima, Nobuo Tsuji, and Hajime Sato, RFP-TRANS-462, translated from The Toshiba Review, 39(7), 611-614, 1984. Laboratory scale vitrification of calcined high-level nuclear wastes using microwave energy was done by the Idaho National Engineering Laboratory, as disclosed in Application of Microwave Energy to Post-Calcination Treatment of High Level Nuclear Wastes, ICP-1183, Allied Chemical Corporation, Idaho National Engineering Laboratory, Idaho Falls, Idaho, Feb., 1979, In the Idaho experiment, high-level wastes were mixed with a composite of fluxing agents in ceramic crucibles and placed in a microwave cavity. The resulting glass was allowed to solidify.
Nevertheless, none of the prior art accomplishes the objectives and achieves the benefits of the invention described below.